This project is a study of the compliant behavior of normal human skeletal muscle studied at three joints: ankle, elbow and wrist. Our intent is to characterize the angle-torque (or length-tension) relationship which is commonly refered to as an elastic one. The static hysteresis properties of the joints suggests that a plastic/elastic relationship is far more appropriate. This means that at any joint angle and torque, there are two different angle-torque relations to consider, one appropriate to incremental lengthening and a different one for incremental shortening. We will also study the dynamic mechanical relations to determine whether even this more complex model of muscle is adequate to more complex, time varying inputs. The study takes as its departure point Fel'dman's experiments on unloading the elbow joint in human subjects and characterizing the behavior as describable as a nonlinear spring with a controllable equilibrium length. Our techniques will use both unloading and loading torques and displacements, interleaved with appropriate instructions to the subjects on how to respond to the perturbations. The results expected to be obtained from these experiments bear directly on current concepts of how the motor control system achieves load compensation (ie reacts to external perturbations) and how it voluntarily controls the limbs to reach desired angles and torques against known external loads.